1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for transmitting and receiving ultrasonic waves to perform imaging of organs, bones, etc. within a living body thereby generating ultrasonic images to be used for diagnosis. Further, the present invention relates to an ultrasonic image boundary extracting method and an ultrasonic image boundary extracting apparatus to be used in the ultrasonic imaging apparatus.
2. Description of a Related Art
In an ultrasonic imaging apparatus to be used for medical diagnoses, an ultrasonic probe including plural ultrasonic transducers having transmitting and receiving functions of ultrasonic waves is used. When an ultrasonic beam formed by synthesizing plural ultrasonic waves is transmitted from such an ultrasonic probe to an object to be inspected, the ultrasonic beam is reflected at a boundary between regions having different acoustic impedances, i.e., between tissues within the object. Thus generated ultrasonic echoes are received and an image is constructed based on the intensity of the ultrasonic echoes, and thereby, the state within the object can be reproduced on a screen.
In such an ultrasonic image, conventionally, it has been attempted to easily and accurately extract outlines (boundaries) of tissues. This is because the extraction of outlines can be utilized for three-dimensional image processing, diagnoses of determination whether a tumor is benign or malignant, and so on.
As a related art, Japanese Patent Application Publication JP-A-8-206117 discloses an ultrasonic imaging apparatus for receiving ultrasonic waves reflected within the object to obtain image data corresponding to each point within a tomographic plane spreading within an object to be inspected in order to objectively extract outlines of tissues without manual operation by an operator or with simple manual operation. The ultrasonic imaging apparatus includes gradient computing means for obtaining gradients of the image data with respect to plural points within the tomographic plane, scalar quantity computing means for obtaining scalar quantities corresponding to the gradients with respect to plural points within the tomographic plane, local maximum point computing means for obtaining plural local maximum points within the tomographic plane where the scalar quantities are local maximums, and outline extracting means for obtaining an outline of a tissue within the object based on the plural local maximum points (page 1, FIG. 1).
JP-P2002-291750A discloses a tumor boundary display apparatus for an ultrasonic image for transmitting ultrasonic waves to the living body and receiving them, and displaying ultrasonic tomographic images within a living body in order to objectively determine a boundary matching vision by approximating a tumor boundary with a polygon and calculating brightness gradients in directions perpendicular to the respective sides. The tumor boundary display apparatus includes tumor boundary approximating means for approximating a visual tumor boundary on the ultrasonic tomographic image with a polygon, selecting means for selecting the respective points inside and outside of the polygon along perpendicular directions from the respective sides of the polygon obtained by the tumor boundary approximating means, brightness gradient extracting means for extracting brightness gradients of the respective points inside and outside of the polygon selected by the selecting means, brightness gradient comparing means for obtaining points indicating the maximum values of rates of change in brightness gradient extracted by the brightness gradient extracting means, and automatically tumor boundary correcting means for determining the points indicating the maximum values obtained by the brightness gradient comparing means as a tumor boundary (page 1, FIG. 3).
JP-P-2000-107183A discloses an organ boundary extracting apparatus including initial point designating means for receiving input of an initial point from an operator, search range setting means for setting a range where a new boundary point is searched for based on a known boundary point, search line setting means for setting a search line from the known boundary point, smoothing means for smoothing of pixel values surrounding the search line, gradient computing means for calculating a derivative value in a search line direction by calculating a gradient on the search line, boundary point determining means for determining a boundary point position based on the derivative value on the search line, and boundary forming means for forming a boundary line from a derived boundary point in order to reduce a possibility of outputting incorrect positions as a boundary from noise or a pixel value distribution similar to an organ boundary by setting a search range according to a shape of a boundary (page 2, FIG. 20).
JP-A-7-8487 discloses an ultrasonic image processing apparatus including a three dimensional data memory for storing echo data retrieved by transmitting and receiving ultrasonic waves in a three-dimensional region within a living body, boundary extracting means for extracting a tissue boundary based on the echo data, and image forming means for forming an ultrasonic image by utilizing the extracted boundary in order to correctly and rapidly perform surface extraction of a tissue for formation of an ultrasonic three-dimensional image (page 2). The boundary extracting means includes first variance value computing means for obtaining variance values by direction of the echo data with respect to each of plural reference directions intersecting at coordinates of interest and three-dimensionally spreading, second variance value computing means for obtaining a boundary value of a pixel of interest by further computing a variance value from the plural variance values by direction, and boundary determining means for determining whether the coordinates of interest is at a boundary point or not.
JP-P2000-126182A discloses a tumor diagnostic method of finding a tumor (especially, mammary tumor) region from a three dimensional image with high precision and automatically extracting determination of malignant tumor with high reproducibility (page 1). The tumor diagnostic method including the steps of quantifying concavo-convex irregularities of a tumor surface shape by defining a parameter S/V-ratio of a ratio of surface area S to volume V of a (benign or malignant) tumor extracted as a three-dimensional image by using a visualizing technology such as an ultrasonic diagnostic method and finding a cancer tissue from normal tissues by extracting boundaries between tissues represented by a three-dimensional image formed by an MRI image, ultrasonic image, or the like of a living body.
On the other hand, recent years, when an ultrasonic image is generated, the use of elements other than intensity of ultrasonic echoes has been studied. It is conceivable that statistical property (statistics values) that represents interrelationship among plural ultrasonic echo signals respectively received by plural ultrasonic transducers is utilized as the elements.
International Publication WO00/40997 discloses that, in order to properly suppress incoherent data in a coherent imaging system, the obtained echo signals are processed along both processing paths of a receive signal processing path using time delays set for traditional coherent receive beam forming and a receive signal processing path using time delays set to apply incoherent summing using time delays equal to zero, for example, and an ultrasonic image is generated based on thus obtained coherent summation signals and incoherent summation signals (page 1). Further, in WO00/40997, an image is generated based on a coherence factor, and displayed as a color map overlaid on a B-mode image. Here, the coherence factor refers to the degree of similarity of a signal that has been phase matched (coherent summed signal A) and a signal that has not been phase matched (incoherent summed signal B), and expressed by the difference between the signal A and signal B, the ratio of the signal A to the signal B, or the like. According to WO00/40997, it can be expected that the image quality of an ultrasonic image may be improved by making a choice among reception signals based on the coherence factor. However, tissue boundaries or angles of reflection surfaces to ultrasonic beams are not obtained.
JP-A-8-117225 discloses a living tissue evaluation apparatus including transmitting means for transmitting ultrasonic waves to a living tissue, intensity distribution obtaining means for obtaining an intensity distribution of ultrasonic waves by receiving ultrasonic waves transmitted through the living tissue and spread, and evaluation value computing means for calculating an evaluation value of the living tissue based on the obtained intensity distribution for analyzing a microscopic structure of the living body by utilizing the intensity distribution of ultrasonic waves transmitted through the living tissue (page 1).
However, in JP-A-8-117225, since an interference phenomenon in transmission is used, information on the depth direction of the ultrasonic beam can not be obtained. Further, any information can not be obtained within objects except for an object within which ultrasonic interference occurs. Furthermore, in JP-A-8-117225, although an intensity distribution among plural reception signals obtained by plural ultrasonic vibrators is obtained and the living tissue is evaluated based on the intensity distribution, boundaries between different tissues are not detected.
JP-A-10-258052 discloses a wave receiving apparatus including a receiver for receiving wave that has reached within an aperture with information on a position within the aperture, a weighting processing unit for performing weighting processing with respective plural kinds of weighting functions using the position within the aperture as a variable, and a computing unit for performing a computation including a computation for obtaining a propagation direction of the wave that has reached within the aperture or a position of a wave source that has generated the wave based on plural weighted reception signals obtained by the weighting processing in the weighting processing unit in order to detect orientation or displacement of a target of detection by one reception with a aperture in an arbitrary position and obtain high resolving power and obtain signals equivalent of reception signals corresponding to an aperture that has not actually received (page 1).
In JP-A-10-258052, although the orientation and position of the target of detection are detected by weighting the reception signals according to the position within the aperture, mutual property and statistics values of reception signals obtained from plural ultrasonic transducers are not utilized.
In all of the above-mentioned JP-A-8-206117, JP-P-2002-291750A, JP-P2000-107183A, JP-A-7-8487 and JP-P2000-126182A, property of image signals such as intensity of image signals (i.e., brightness), derivative values thereof, variances, and statistics values are used as data for extracting boundaries. Since the image signals are generated based on the intensity of ultrasonic echoes obtained by reflection of ultrasonic waves at boundaries of tissues, they are susceptible to various factors such as surface property of tissues (irregularities and hardness/softness). Accordingly, even when boundaries are detected by utilizing such image signals, sometimes precision is low and tissues other than boundaries are incorrectly detected. Further, the detection precision often becomes unstable due to gradation of brightness or the like.
Further, purposes of the above-mentioned WO00/40997, JP-8-117225 and JP-A-10-258052 are to exclusively improve image quality of ultrasonic images, and extraction of tissue boundaries is not performed.